The present invention relates to a vehicle power steering system, and more specifically to a power steering system having a pump which supplies fluid through a control valve to a steering motor.
Many power steering systems have fixed displacement pumps which are continuously driven by an engine. The displacement of the pump is selected so that the pump can satisfy the peak demands of the steering system. When the power steering system is in an inactive condition, the fluid output from the pump is merely recirculated. Upon actuation of a steering control valve, fluid from the pump is ported to a steering motor connected with steerable vehicle wheels. Such systems are inefficient, since the steering pump is continuously driven by the engine at a relatively high fixed displacement.
Other known steering systems include a variable displacement pump. The pump displacement is increased to provide an increase flow of steering fluid in response to a demand for steering fluid in order to maintain a load compensated stand-by pressure in the system. U.S. Pat. Nos. 2,892,311 and 2,892,312 disclose such systems. However, such known systems are not of the type which include a follow-up mechanism interconnected between the vehicle steering wheel and the wheels to effect turning of the wheels in accordance with turning of the steering wheel. Examples of such follow-up mechanisms are hydrostatic steering devices, as shown in U.S. Pat. No. 3,452,543 and intergal steering gear similar to that shown in U.S. Pat. No. 3,606,819. Further, such systems do not include a metering mechanism as in hydrostatic steering systems and which is operable in response to manual steering to demand fluid from the pump and which directs a metered fluid flow in accordance with the rate of operation thereof to the steering motor. The advantages of such metering mechanisms are well known and, in general, provide for a proper follow-up action between steering wheel turning and vehicle turning. To applicant's knowledge, no one has heretofore provided a system where steering fluid flow and pressure are supplied and regulated to that level demanded by the steering control and which system is constructed to provide the desired follow-up action between steering wheel turning and vehicle turning.
More specifically, the present invention includes a variable displacement pump which supplies fluid to the steering system and a valve which ports that fluid to the steering motor to effect vehicle steering. Incorporated in the system is a follow-up mechanism which is operable upon vehicle steering and which demands fluid as it is operated. In response to the demand for fluid, the displacement of the pump is increased to increase the pump output so that fluid is delivered to the steering motor in accordance with demand and at a modulated load compensated standby pressure sufficient to effect steering action.